Speech encoding schemes having scalable function (function whereby decoding from partial encoded data is possible on the receiving end) are suitable for traffic control of speech data communications and multicast communications on IP (Internet Protocol) networks. The CELP encoding scheme is a speech encoding scheme enabling high sound quality at a low bit rate, and adjustment of sound quality according to the bit rate is possible by being applied to a scalable encoding scheme.
In CELP encoding of a speech signal, the adaptive codebook (ACB) search (an excitation search employing a past excitation signal, i.e. the adaptive codebook) will have an effect on the sound quality of the encoded speech signal and on the bit rate needed for transmission thereof. In scalable CELP encoding, the effects thereof further increases. Moreover, in scalable CELP encoding, while encoding schemes that do not employ an enhancement layer for an adaptive codebook are known (see, for example, FIG. 3 of Non-Patent Document 1), the use of an adaptive codebook provides generally good sound quality of the encoded speech signal, since past excitation signals continually-updated for optimization can be utilized effectively (see, for example, FIG. 5 of Non-Patent Document 1).
FIG. 1 shows the temporal relationship between a sub-frame targeted for encoding, and the section of the adaptive codebook searched to generate an enhancement layer adaptive excitation candidate vector for the sub-frame targeted for encoding, in the case of an excitation search carried out during CELP encoding for each sub-frame in the enhancement layer. As shown in FIG. 1, the enhancement layer adaptive excitation candidate vector is retrieved by searching a prescribed section of the adaptive codebook, which is an integration of excitation signals preceding in time the sub-frame targeted for encoding in the enhancement layer. The adaptive codebook in the enhancement layer is generated and updated by the following procedure.
(1) Encoding of core layer
(2) An adaptive codebook search (pitch prediction) is carried out in the enhancement layer using the core layer excitation, the adaptive excitation lag (pitch cycle T0) of the core layer and the adaptive codebook of the enhancement layer (auxiliary adaptive codebook), and an adaptive excitation is generated from the adaptive codebook(3) A fixed excitation search and gain encoding are carried out in the enhancement layer(4) The adaptive codebook of the enhancement layer is updated using the encoded enhancement layer excitation signal derived through (1) to (3) above.Non-Patent Document 1: Journal of IEICE, D-II, March 2003, Vol. J86-D-II (No. 3), p. 379-387